Why Earth’s Accelerating Spin Might Lead to a Historic ‘Negative Leap Second’

The Earth has been spinning faster lately. Last year, on July 4, it completed a full rotation 1.66 milliseconds faster than usual, as reported by timeanddate.com. Fast forward to July 10, 2025, and scientists noted another short day, this time 1.36 milliseconds quicker. Other days in July also showed minor reductions in time, although precise figures are still under review here.

While losing a few milliseconds might seem trivial, it can throw off systems that rely on precise timing—think GPS, banking, and communication networks. Atomic clocks keep our global time standard via Coordinated Universal Time (UTC). With the Earth’s rotation speeding up, some experts are discussing the idea of adding a “negative” leap second.

Leap seconds adjust UTC to align with astronomical time, which is based on how quickly the Earth spins. A typical day has 86,400 seconds, but various factors, such as the Sun’s position and the Moon’s influence, can cause irregularities. This inconsistency between UTC and solar time can create a need for corrections via leap seconds.

Some experts argue that introducing a negative leap second could help manage these discrepancies. However, this proposal isn’t without controversy. In 2020, a global expert group voted to phase out leap seconds altogether by 2035, due to issues they cause in computing systems, especially with international flights Live Science.

Patrizia Tavella from the International Bureau of Weights and Measures noted that leap seconds often lead to “failures and anomalies” in tech. Countries also handle leap seconds differently, which complicates global operations even further.

Opponents of the negative leap second caution that this change could also cause confusion. As Darryl Veitch, a computer networking expert, pointed out, inserting a negative leap second into our timekeeping might pose challenges due to our interconnected world. Judah Levine, a physicist, echoed these concerns, stating that problems have persisted with positive leap seconds for decades, raising doubts about implementing a negative one.

The chances of adopting a negative leap second seem slim, especially since scientists are already working to retire the positive version. Yet, as the Earth continues to rotate more quickly, astronomical time may eventually lag, prompting a re-evaluation of this idea. Levine estimates a 30% likelihood of this needing to happen in the next decade. Interestingly, oceanographer Duncan Carr Agnew believes it could occur even sooner, by 2029.

Ultimately, the future remains uncertain. Will the Earth slow down again, or continue spinning faster? Time will tell, and we can expect another “short” day on August 5. Keeping an eye on these changes could be fascinating!

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